1. Field of the Invention
The present invention relates generally to a wafer polishing apparatus, and more particularly to a wafer polishing apparatus with a chemical mechanical polishing method (CMP).
2. Description of Related Art
A wafer polishing apparatus with a CMP polishes an oxidized membrane and a metal membrane, and so forth, which are formed on a wafer, and such a polishing apparatus requires a precise control of a polishing amount. Thus, as an apparatus for controlling the polishing amount, that is, as an apparatus for detecting an end of the polishing, an apparatus is suggested which determines a rotation torque of a wafer holding head and then detects the end of the polishing according to the rotation torque.
As shown in FIG. 4, the apparatus for detecting the end of the polishing detects, focussing on a difference in rotation resistance (processing resistance) which occurs due to different processing resistances at layers and different types of polishing liquids (slurries), the change in the processing resistance and finishes processing at an intended layer. The relationship between the each layer and the rotation resistance is "metal layer"&lt;"membrane between layers"&lt;"barrier metal layer".
However, because the apparatus for determining the rotation torque of the wafer holding head according to a change in electrical resistance of a motor determines the rotation torque including the rotation resistance of a deceleration mechanism connecting an output shaft of the motor and the wafer holding head, the rotation torque of only the wafer holding head cannot be determined exclusively; thus the end of the polishing cannot be detected accurately.
A wafer polishing apparatus disclosed in Japanese Patent Application Laid-open No. 9-19863 forms a pressurized air layer between a carrier and a wafer, and presses the wafer against a platen through the pressurized air layer to polish the wafer. Moreover, according to the wafer polishing apparatus, a retainer ring is arranged at an outer periphery of the carrier; thus, the retainer ring prevents the wafer from jumping out of the carrier during polishing. In other words, the outer periphery of the wafer during polishing is contacted with an inner periphery of the retainer ring whereby preventing the wafer from jumping out of the carrier.
Directed to a case in which the retainer ring is directly fixed to the outer periphery of the carrier, an impact at contact of the wafer to the retainer ring is not absorbed to the retainer ring side, and such an impact rather becomes a counterforce applied to the wafer; as a result a wafer may be damaged.
In view of the above-described adverse effect, the wafer polishing apparatus has the retainer ring arranged with a gap with respect to the outer periphery of the carrier, so as to prevent the wafer from a damage by absorbing the impact with the gap.
Further, the wafer polishing apparatus has the retainer ring arranged with the gap with respect to the carrier; thus the wafer is polished, its position with respect to the carrier always moving.
In the wafer polishing apparatus, the wafer is polished preferably in a state that the center of the wafer is held on the central axis of the carrier in purpose of improving the polishing precision of the wafer.
However, the conventional wafer polishing apparatus has a difficulty in improving its polishing precision of the wafer since the position of the wafer always changes.
This problem can be solved by fixing the retainer ring directly to the outer periphery of the carrier and polishing the wafer without the gap, but the wafer may be damaged without the gap as described above.